In the Linear Tape Open (LTO) tape drives, data packets (each of which includes a block of data and related overhead) are written on tape tracks in structures known as synchronized codeword objects (SCO). SCOs consist of two encoded data units, each with an associated header, and added synchronization patterns. The encoded data unit consists of m interleaved Reed-Solomon codewords, designated by the term Codeword Interleave-m (“CWI-m”). In LTO generations 1 to 4, m=2 and SCOs consist of two CWI-2s with the associated headers and added synchronization patterns. In LTO, a structure of two CWI-m's with the associated headers is referred to as a codeword object (“CO”). In LTO-1 to LTO-4, the minimum unit that can be written is a set of T COs, where T is the number of physical tracks on the tape that are written simultaneously.
During magnetic tape recording, errors may be detected by using a read-while-write process in which data is read and verified immediately after being written. If an error is detected, the tape drive rewrites the same data to ensure that at least one correct instance of the data exists on the tape. In the LTO 1-4 standard, because the minimum unit that can be written is a CO set, the minimum unit that can be rewritten is also a CO set. In future LTO generations, format efficiency will be improved by increasing the length of the codeword object. However, the penalty for rewriting codeword objects will increase in a corresponding manner.
For example, in a tape cartridge having an actual capacity of 1680 GB, an average of 5% of the capacity, or approximately 80 GB, may be reserved for rewriting CWI-2s, resulting in a nominal (useful) capacity of 1600 GB. If the number of SCOs which are rewritten remains the same, the space needed for the rewrites doubles to approximately 160 GB, reducing the nominal capacity of the cartridge by approximately another 80 GB to 1680−160=1520 GB. Such a loss in cartridge capacity is significant.